Here are some tips,
Alk and Ca, along with Mg needs to be as constant as possible.
If your 20K lamps are less than 10 months old, hold off on buying new bulbs and purchase a Ca reactor instead.
The Ca reactor will stabilize your Ca/Alk/Mg uptake and will immediately improve your SPS colonies.
The Hydno can cause problems, but I doubt it has anything to do with your growth problem. It is related to one of two things. Either you are exhausting available Ca and trace elements, or you have a higher level of PO4 than you realize. (Only a Photometer will give you a accurate reading)
Your corals have good color, so PO4 is probably not the issue as it would brown out so the addition of a Ca reactor should help immensely.
As lighting goes, 10K bulbs have shown to be the best for rapid growth, however color may suffer compared to 20K bulbs. 20K tend to fluoresce corals more and make the colors stand out more. If you are interested in rapid growth for frags, I suggest the 10K's, if you want good colar and nice display, I suggest the staying with 20K's. If your bulbs are older than 10 months change them.
Flow is very underestimated. By adding some high volume powerheads like Koralia 4 you can increase flow and in turn stimulate growth. If you already have these powerheads, you can move them around to create a new flow pattern that will also stimulate the coral's growth.
Most importantly, make sure you are doing regular water changes. If there is a water parameter problem this will help to fix it.
Also, if you do change the bulbs, remember to light acclimate your tank by raising the lamps for the first 2 weeks or use a shorter photoperiod so that you do not bleach your corals.
~Rich
we'll see.
